Solid sealant with environmentally preferable corrosion resistance

ABSTRACT

The faying surface of one or of both of a pair of substrates is provided with a layer of solid polyurethane sealant which is adherent to at least one of the respective surfaces. The substrates are additionally fastened together so that the faying surfaces compress the polyurethane sealant between them to form a fluid seal across the entire spacing between the two surfaces, and between the polyurethane sealant and the faying surfaces themselves.

FIELD OF THE INVENTION

[0001] Sealing against passage of fluids—gases or liquids—between twofaying surfaces which confront one another with a layer or layers ofsolid sealant in contiguous and continuous fluid-sealing contact withthe surfaces, which include an environmentally preferable corrosionresistance component.

BACKGROUND OF THE INVENTION

[0002] Sealing against the passage of fluids—gases or liquids—betweenfaying surfaces is a very old art. “Faying” surfaces are those whichdirectly abut each other, or which almost abut each other with anintermediate body between them such as a sealant. Built-up structuresare replete with faying surfaces. The substrates bearing the surfacesare customarily joined together by fasteners such as rivets or threadedcombinations whose purpose is to hold them against separation and tolimit or prevent relative shear movement between them.

[0003] Especially in structures which are subject to a variety ofenvironments such as substantial temperature change, and deflection dueto bending or vibration, relative movement and separation between localregions of confronting faying surfaces cannot be completely prevented.Thus even those flat surfaces which are practically attainable whenbrought together will in use permit leakage of fluid between them.

[0004] This is a tolerable situation where the confinement of a fluid isnot required. Many faying surfaces are joined without regard forsealing. However in applications such as fuel tanks, leakage of fuel isnot tolerable. In aircraft fuselages, leakage of air from a pressurizedcabin must at least be minimized. There are numerous other examples inaircraft and spacecraft as well as in ground based structures wherefluid containment (gas or liquid) in a built-up structure is required.

[0005] This is an old problem, and it has been solved in various wayswhich utilize sealants applied to appropriate parts of the structure.With rivets or fasteners in areas where sealing is critical, beads ofsealant are applied around exposed edges of adjacent surfaces, often inthe form of wet sealants which cure or dry after assembly. Thetechniques for application and subsequent clean-up are both expensiveand labor intensive, and complicate the assembly, maintenance and repairof the structure.

[0006] In aircraft construction, a wet layer of sealant is often placedbetween the faying surfaces at the time they are joined together, andthe sealant cures after the assembly is completed. The process isdesigned in such a way that the entire interstitial area will be filled.To assure this, an excess of sealant is applied before the surfaces arebrought together. Some of the sealant is expelled from between thesurfaces when they are joined, and their excess sealant must be removedfrom the adjacent area around the sealed edges. The problems created bythe excess sealant are not trifling. The surrounding area becomes a messthat has to be cleaned up, and all excess sealant must be carefullyremoved. If the sealant contains toxic additives such as chromates, thetoxic excess requires careful disposal methods which are expensive. Theexpensive disposal extends to auxiliary items such as the cloth used forremoval, brushes used for application, and the like. Worker protectionmust be provided against contact with such a sealant, requiring the useof masks and gloves. Solvents such as 1,1,1-trichloroethane and ethersolvents which are used for the cleanup bring their own hazards.

[0007] In order to assure adequate filling of the interstitial region,it is not sufficient merely to provide an excess of sealant. It is alsonecessary to provide a uniform excess. This requires a further toolingstep to rake and trim the exposed wet surface to a uniformly thick areaof a configured shape.

[0008] These labor intensive procedures are costly. They must beperformed a reasonably short time before assembly so the sealant remainsfluid while the assembly is completed. This is a serious limitation onthe freedom to schedule production, because the surfaces cannot beprepared long before assembly and then wait their turn for use.

[0009] The above labor and economic problems and shortcomings of asealant which is applied wet at the time of assembly are severe. Theyalso involve the economic problems of material waste and structuralweight penalties. The waste of expensive sealant material which must bewiped up and disposed of is the lesser of these.

[0010] Of far greater consequence is the weight penalty. It has beencalculated that on a large aircraft such as the C-17, the use of the drysealant system of this invention can reduce the total sealant weightrequired by about 800 pounds compared to the weight of a wet sealanteven when the wet sealant is applied in an optimum manner. It should beremembered that weight is an extremely expensive quantity in aircraftand spacecraft, because each pound and structure to support it requiresfuel to raise it every time it is lifted. It has been estimated that inaircraft, each pound costs about hundreds of dollars over the usefullife of the aircraft.

[0011] A sealant which can be applied well before assembly and handledwhile dry can be made to closer tolerances, without applying excesssealant to assure that there is enough. This potentially avoids the mosttroublesome and costly problems. In addition, the part can be preparedlong before it is needed for assembly, and can be used when it is mostconvenient to the production schedule.

[0012] However, attempts to coat one or both of the surfaces, drying thecoating, and then joining them has not previously been successful. Thereasons reside in the stringent conditions the sealant layer mustfulfill.

[0013] To be successful for its intended purpose the sealant must be dryso that it can physically be,,handled without changing the shape of thesealant layer, or fouling the surroundings. It must not extrude tobecome a nuisance after assembly, and the sealant must ultimately comeinto complete conformity with both surfaces. The surface to which it isdirectly applied will assuredly be fully abutted. However, the exposedother surface of the dry sealant must effectively contact and engage theother surface (or the exposed surface of an opposite sealant layer).Accordingly, in the substantial total thickness required for a practicalsealant, often bridging surfaces from between about 0.005 and 0.01inches apart, down to near contiguity, the dry sealant must bedeformable, but not be liable to substantial cold flow. This is assuredby control of the physical properties of the cured sealant.

[0014] In order to be practical, the thickness of the dry layer must beconsistent and readily applied. Surfaces to be covered come in a widerange of sizes and configurations, from long spars and wide panels tointersections with tight corners. While techniques such as spraying,brushing, rolling and flooding can in many situations effectively beused, in general from the point of view of production efficiency,spraying is the preferred method. For this reason sprayable coatings arethe most desirable manner of application.

[0015] In order to be practical for production purposes the uncuredsealant should not contain any solvents or volatile materials (althoughin some cases the use of water as a solvent might be acceptable). Ifsolvents are present they present ventilation problems, as well as fireand explosion hazards. In addition solvent evaporation from the film canlead to pinholing and film shrinkage with subsequent possibledevelopment of leaks.

[0016] The sealant must be strongly adherent to its substrate and becompatible with both its substrate, with other sealant compounds whichare used to form fillets or beads, and with tackifiers and otheradhesives if they are used. Such other compounds are characteristicallyapplied as a backup as reassurance against leakage through the spacingbetween the faying surfaces, and to resist corrosion when a corrosionresistant substance is provided as an additive.

[0017] While a heat curable sealant is useful in this invention, theapplication of curing temperatures to many substrates could causewarpage or other damage to a substrate such as an aircraft panel. It ispreferable for the sealant to cure at room temperature, for examplebetween about 60 degrees F. and about 120 degrees F.

[0018] A substantial pot life is desirable when a pre-mixed liquidsealant is applied. The term “pot life” is less meaningful if thesealant is a multiple component mixture that is mixed in a dispensingnozzle. In this case cure time after application is a more appropriateterm. While cure times can vary from minutes to a week or so, a curetime not much longer than about 16 hours is most practical in amanufacturing venue. An overnight cure of about 16 hours is about aslong as a manufacturing operation is likely to tolerate. After the cureis completed, there should be no limitation on how long the sealant mayremain exposed. Certainly it should not be so long as to require a largenumber of parts to be treated in advance and held in inventory while thesealant cures.

[0019] The long list of constraints continues with the requirement thatthe sealant resist corrosion and solvent attack by many commonsubstances. These substances include such frequently-encounteredexamples as air, water 1,1,1-trichlorethylene, halogenated hydrocarbons, aromatic solvents such as toluene, common solvents such asketones (MEK), esters (butyl acetate), alcohols (methyl and ethyl), andhydrocarbon fuels such as JP8.

[0020] Production of a continuous and uniform sealant layer isessential. For example, some polymer systems are very sensitive to thepresence of water, which can generate void inclusions. The sealant mustbe readily mixed in convenient apparatus in a conventional manufacturingenvironment.

[0021] Other requirements, especially for aerospace operations in whichthe use of this invention will be most frequent is the ability towithstand and operate over a wide range of temperatures, generallybetween about −65 degrees F. and about 250 degrees F. or wider. Thecapacity to be repaired if damaged is essential. Suitability for repairrequires that a later-applied application of the sealant can form acontinuous bond with a contiguous remaining layer of undamaged sealant.

[0022] In view of this array of requirements, which steadily become moredemanding as the complexity, size, and ambient and physical conditionsbecome more severe, it is not surprising that the concept of utilizingdry sealant layers to seal between faying structures has been neglected.The use of more expensive, complicated, and labor intensive sealingtechniques utilizing wet sealants which are later cured in place havebecome the accepted mode despite their cost and other disadvantages.

[0023] It is another object of this invention to simplify and reduce thecost of a reliable seal between faying surfaces, at the same timeproviding one which is more reliable and much less likely to requirerepair.

[0024] It is yet another object of this invention to provide structurecomprising a pair of assembled substrates with faying surfaces bridgedby a cured sealant layer according to this invention.

[0025] Sealants of this type are often applied to surfaces that aresubject to corrosion by the environments in which they are used. Metalaircraft structures made of materials such as aluminum, titanium andcomposites are examples. To counter this risk, sealants customarilyinclude a corrosion resistant component. By far the most extensivelyused substances for this purpose are metal chromates, particularlystrontium chromate, zinc chromate, and barium chromate, and theirmixtures. These function well for this purpose, and heretofore haveenjoyed widespread and usually uncritical acceptance.

[0026] However, chromates themselves have become environmentallyobjectionable. Their handling in manufacturing and disposal operationshas become more regulated and troublesome. In some applications the merepresence of chromates per se has become an issue. Still, the protectionof surfaces intended to remain in service for many years, even decades,and in which there is very limited access for inspection, and repair isexcessively expensive even where it is possible, the use of adequatecorrosion resistance components is essential. For this purpose chromatesare much to be preferred, their effectiveness for very extending periodsof time having been proved long ago.

[0027] It is another object of this invention to provide a non-chromatecorrosion resistant element for sealants, especially effective forsealants between faying surfaces, but also effective in other types ofcoatings as well.

BRIEF DESCRIPTION OF THE INVENTION

[0028] This invention provides the faying surface of one or of both of apair of substrates with a layer of solid sealant which is adherent to atleast one of the respective surfaces. The substrates are fastenedtogether by mechanical means so the faying surfaces compress the sealantbetween them to form a fluid seal across the entire spacing between thetwo surfaces, and between the sealant and the faying surfacesthemselves.

[0029] According to this invention the sealant consists essentially ofan organic polymer having the following physical properties:

[0030] (1) Sprayability in its liquid pre-cured condition, and theability to cure to a solid at room temperatures.

[0031] (2) Resistance to deterioration and solvent attack, and abilityto seal against passage through it, and between it and the fayingsurfaces, of air, water, common solvents and hydrocarbon fuels.

[0032] (3) Temperature tolerance to resist thermal decomposition, and toprovide sealing properties at temperatures between about −65 degrees F.and about 250 degrees F.

[0033] (4) Compatibility with substrate surfaces of metal such asaluminum alloys, titanium alloys, steels, and structural compositematerials.

[0034] (5) Adequate deformability to accommodate surface irregularitiesin the substrates which may be exasperated by temperature excursions anddeformation.

[0035] (6) Compatibility with sealants customarily used to form beadsand fillets.

[0036] (7) Ability to join and form a continuous seal with the edges ofa previously deposited and cured layer of a similar or identical sealantmaterial.

[0037] (8) Inclusion of a corrosion resistant component which is not achromate.

[0038] A sealant for use with this invention comprises an organicpolymer possessing the foregoing physical and chemical properties, inwhich the physical properties are derived from a molecular structurewhich includes three-dimensional cross-linkage to form a structurallattice to enhance heat and solvent resistance. Numerouspolymeric,systems may be employed, including polyurethanes, polyesters,epoxies, acrylics, synthetic rubbers, and natural rubbers. While all mayfind utility in many or most applications, the polyurethanes involve thefewest difficulties in their preparation and application, which are aswell to avoid. Further, they may readily formulated to provide the abovefeatures over a wide range of values. Accordingly, while other systemsfall within the scope of this invention, polyurethanes are much to bepreferred, and will be emphasized in this specification. A corrosionresistant component, a borate, preferably zinc borate, is included.

[0039] A structural assembly according to this invention comprises apair of substrates mechanically joined with their faying surfacesbridged by a layer of said sealant.

[0040] The above and other features of this invention will be fullyunderstood from the following detailed description and the accompanyingdrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 is a fragmentary plan view of an assembly of fayingsurfaces and sealant according to this invention; and

[0042]FIG. 2 is a cross-section taken at line 2-2 in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0043] In the drawings a pair of substrates 10. 11 having fayingsurfaces 12, 13, are joined together by a fasteners 14 or othermechanical means. A rivet is shown. Instead threaded pin-collarassemblies could readily be used. A layer 15 of sealant according tothis invention is placed between them to seal the spacing 16 betweenthem.

[0044] The substrates often will be metal sheets or flanges, formed ofaluminum alloys, titanium alloys, or steels. The fasteners may be of anysuitable type such as rivets and threaded combinations of pins andcollars. The identity of the substrates and fasteners are notlimitations on the invention, but are given only as illustrativeexamples of the uses to which this invention can be put.

[0045] While a layer of sealant can be applied to both surfaces 12 and13, this will usually be an unnecessary duplication of labor. If a layeris applied to both surfaces, each must be thinner than a layer appliedto only one surface. In that event, each layer will be contiguous to andadherent to its own respective surface. After assembly, the exposedsurface of each layer will conform to the exposed surface of the otherto complete the seal, instead of conforming to the other faying surface.This construction, while not preferred, is within the scope of thisinvention. It is in the nature of this sealant to make a closedcompressive seal. While the previously exposed surfaces of the sealantwill not ordinarily fuse with each other, their mutual resistance toflow and common deformability will assure a fluid-tight seal betweenthem.

[0046] When only one layer is used on only one of the faying surfaces,the sealant adheres to its respective surface, and is pressed againstthe other surface by the assembly procedures. The cured sealant materialresists cold flow, and feels slightly soft, preferably having a Shore Ahardness between about 30 and about 70. This feature aids in maintaininga prevailing sealing force. In modern compressive rivet practice in theaircraft the sealant will usually be compressed between the substrates,and then the rivet will be driven. While the rivet may thereafterelongate a bit after the setting forces are removed, it still willmaintain some compression on the sealant. When a threaded fastener isused, it will maintain a prevailing compression force on the substrates,and thereby on the sealant.

[0047] Each of the requisite properties of the sealant, both before andafter curing, has a substantial range of acceptability. The task ofdesigning the sealant therefore becomes one of providing it with each ofthe properties within the acceptable range. Evidently there can be atleast several sealant compositions whose properties will fall within theselected limit ranges. Any of these can be used, but the selection amongthem will often be determined by their convenience in formulation anduse, and in minimized requirements for control of environmentalconditions during mixing and application, and of course minimumtoxicity.

[0048] Although other systems are useful and acceptable, generally theurethanes will be much preferred.

[0049] Although isocyanates are generally toxic and many monomericisocyanates are quite volatile, their product of reaction with polyolsis much less toxic and readily dealt with. Polyurethanes cured fromsuitable starting materials can be completely acceptable, and are thepreferred embodiment of this invention.

[0050] As design (selection) criteria, for solvent resistance,resistance to cold flow under the anticipated temperature conditions,and proper hardness, a suitable amount of cross-linking is necessary.However, if the cross-linking is too great, the cured material will betoo hard and too brittle for use. If there is insufficient cross-linkingthere will be insufficient resistance to solvents or cold flow.

[0051] In order to formulate a sealant with the desirable properties asdescribed above it is necessary to strike a proper balance between theamount of cross linking of the final polymer and the chain lengthsbetween cross links in the backbone of the molecule. Too much crosslinkage leads to hard, brittle polymers with insufficient flexibility toperform the required sealing as described above. Too little crosslinking leads to polymers which show poor solvent and chemicalresistance as well as poor resistance to cold flow or creep.

[0052] Cross linking is attained in polyurethanes by usingmulti-functional monomeric starting materials (isocyanates or polyols).In this case multi functional is defined as a functionality greater thantwo. In order to produce a long chain polymer both the isocyanate andpolyol materials must have a functionality of at least two.

[0053] In addition to sufficient cross-linking, the choice of chainlength is important for the properties of softness and flexibility. Hereone considers that if long chain monomers are used to provide softnessand flexibility, they must be highly functional. If shorter chainmonomers are used, the functionality can be decreased, but there stillmust be sufficient chain length for the cured sealant to have thedesired physical properties.

[0054] There are many possible routes to producing a polymer with thecorrect balance or cross linking and chain length. For example longchain diols can be cross linked with suitable amounts of short chaintriols (or tetraols, etc.). Conversely long chain triols can be used tocross link short or medium chain length diols.

[0055] With the foregoing in mind, the formulator of sealants accordingto this invention will select appropriate chain lengths andfunctionalities, and mix the reactants prior to application, or mix themas they are being applied, perhaps in a spray gun.

[0056] Adjustments to the various properties may be made by selectinglonger or shorter chains and greater or lesser functionalities.

[0057] Additives for various purposes may be including in the pre-curedmix, for example corrosion resistant compounds and catalysts.

[0058] Formulations according to this invention do not use chromates forcorrosion resistance. Instead borates, preferably zinc borate, will beused for this purpose. Zinc borate in amounts between about 3% and 30%by weight of the formulation is useful. Its preferred range is betweenabout 6% and about 12% by weight. Percentages less than about 6% areuseful, but at least that amount is to be preferred. Amounts above about12% do not appear to offer enough greater effect to justify their use.About 6%-8% will generally be seleQted within the preferred. range.

[0059] Conventional catalysts may be used. Organic metal salts,especially salts of tin, and mercury are frequently used. Amines arealso useful catalysts. Tertiary amines provide for a fast cure that isdifficult to control. Secondary amines do not result in a fast a cureand are well-regarded for the purpose. The most commonly used aminecatalysts are primary amines.

[0060] Any corrosion resistant additive and any catalyst which is notdeleterious to the composition is within the scope of this invention.The above being merely the preferred examples.

[0061] With the foregoing in mind, the following illustrative examplesare given. The preferred polymer system is a polyurethane.

[0062] The various polyols can be obtained as urethane grade materialsfrom a variety of suppliers. The following table shows a few examples ofsome of the commercially available materials. This table is not intendedto be complete, but only shows a sample of the wide variety of availablematerials. Functionality Approx. Product Name Supplier (Type) Mol. Wt.Multranol 9121 Bayer 2 (diol) 425 Poly G 20-265 Olin 2 (diol) 425 Poly G20-112 Olin 2 (diol) 1000 Multranol 9109 Bayer 2 (diol) 1000 Multranol3600 Bayer 2 (diol) 2004 Poly G 20-56 Olin 2 (diol) 2000 Poly G 20-28Olin 2 (diol) 4000 Multranol 9195 Bayer 2 (diol) 4000 Multranol 9133Bayer 3 (triol) 160 Poly G 70-600 Olin 3 (triol) 282 Poly G 30-280 Olin3 (triol) 615 Multranol 9157 Bayer 3 (triol) 673 Multranol 9144 Bayer 3(triol) 1122 Poly G 30-168 Olin 3 (triol) 1000 Poly G 30-112 Olin 3(triol) 1500 Multranol 9180 Bayer 3 (triol) 1503 Multranol 9187 Bayer 3(triol) 2805 Poly G 30-56 Olin 3 (triol) 3000 Poly G 30-42 Olin 3(triol) 4000 Multranol 9168 Bayer 3 (triol) 3740 Multranol 9181 Bayer 4(tetraol) 291 Multranol 9173 Bayer 5.5 671 Multranol 9185 Bayer 6 3366

[0063] The above products are well-known. Their features which areimportant to this invention are shown in the foregoing table. “Bayer:refers to Bayer Corporation, 100 Bayer Road, Pittsburgh, Pa. 15205-9741.“Olin” refers to Olin Industries.

[0064] By combining the proper mixture of high and low molecular weightpolyols, and by using a variety of ratios of dials to polyols withfunctionalities greater than 2, a variety of urethane polymers withdifferent degrees of cross linking and various physical properties canbe obtained. For example backbone chains prepared from high molecularweight diols (to give flexibility) can be cross linked with lowmolecular weight triols to result in urethane polymers with the desiredphysical properties and chemical resistance. Conversely lower molecularweight dials can be cross linked with higher molecular weight triolsand/or tetraols to also obtain a desirable combination of properties.

[0065] The above discussion has concerned ways of formulatingpolyrethanes with the desired properties by the proper choice of hydroxycompounds. An equally powerful method of obtaining a variety ofproperties can be used by choosing the prober polyisocyanate. However inpractice the properties of the polymer are generally determined by thechoice of hydroxyl compounds, and the isocyanates are chosen for otherconsiderations.

[0066] It has been found that in general the strongest but leastflexible polymers result when aromatic isocyanates are used for theirpreparation. Conversely more flexible but less heat resistant polymersresult when aliphatic isocyanates are used for their preparation.Intermediate properties are obtained when cycloaliphatics are used. Evenmore important than the influence of physical properties by theisocyanates is the resistance to yellowing and weathering when thepolymers are exposed to ultra violet radiation such as occurs in normaloutdoor exposure. Polyurethanes prepared from aromatic isocyanates showpoor resistance to weathering and yellowing whereas those prepared withaliphatic or cycloaliphatic isocyanates exhibit good weatheringcharacteristics.

[0067] Another important consideration when choosing the isocyanate usedfor making a polyurethane is raw material cost. In general the aromaticisocyanates are the least expensive and the cycloaliphatic ones are themost costly.

[0068] As stated above, isocyanates are generally toxic, and thearomatic isocyanates are more toxic than the other types. The primarydanger encountered when working with isocyanates is from inhalation ofthe vapors. The risk from exposure to vapors can be dramatically reducedif instead of using monomeric isocyanates, their volatility is reducedby using them in a polymeric form. Polyisocyanates are commerciallyavailable as are isocyanate terminated prepolymers. These materials canbe used as substitutes for all or part of the monomeric isocyanates forthe preparation of polyurethanes.

[0069] As with the hydroxy compounds, isocyanate materials especiallymanufactured for the preparation of polyurethanes are commerciallyavailable from a variety of manufacturers. A few of the suitablematerials available from Bayer are listed below.

[0070] Mondur ML: Aromatic monomeric diisocyanate.

[0071] Mondur TDS: Aromatic monomeric diisocyanate.

[0072] Desmodur W: Cycloaliphatic monomeric diisocyanate.

[0073] Mondur MR: Aromatic polymeric diisocyanate.

[0074] Baytec ME-040: Isocyanate terminated polyether prepolymer.

[0075] Baytec ME-090: Isocyanate terminated polyether prepolymer.

[0076] Baytec MS-041: Isocyanate terminated polyester prepolymer.

[0077] Baytec WE-180: Isocyanate terminated aliphatic prepolymer.

[0078] In addition the aliphatic hexamethylene diisocyanate can bepurchased commercially.

[0079] The foregoing isocyanates and polyols may be obtained from theBayer Corporation. Further information regarding them will be found inits publication entitled “Polyurethane Rat Materials ProductIndex—Polyurethane Products”, copyright 1996 which is incorporatedherein in its entirety in this invention for such information, and acopy is being filed along with this application.

[0080] The general method for formulating practical polyurethanes is tofirst choose the isocyanate portion of the composition based first onthe requirements of resistance to weathering and then on the otherfactors such as cost, toxicity, method of application of the finalcomposition etc. Once the isocyanate has been chosen, the desirablephysical properties of the final polymer are obtained by the properchoice of the hydroxy components as described above.

[0081] The following are examples of suitable formulation of polyols andisocyanates together with other ingredients, which when mixed will cureto form a useful sealant according to this invention in a suitableperiod of time. Examples 1-6 are urethane systems. Percentages are byweight. The polyols and isocyanates are more completely described in theforegoing lists. DBTDL identifies dibutyl tin dilaurate, which isprovided as a catalyst. Example 1 Poly G 20-56 38.85% Multranol 910938.85% Mondur MR 16.3% DBTDL 0.01% Zinc Borate 6% Example 2 Poly G 20-5670.7% Poly G 70-600 6.7% Mondur FL 18.6% DBTDL 0.01% Zinc Borate 4.0%Example 3 Multranol 9109 47.1% Poly G 30-280 19.3% MRS-4 25.6% DBTDL0.01% Zinc Borate 8% Example 4 Multranol 9195 76.8% Multranol 9133 4.1%Mondur MR 16.1% DBTDL 0.015% Zinc Borate 3% Example 5 Poly G 20-56 44.8%Multranol 9185 25.1% MRS-4 12.1% DBTDL 0.01% Zinc Borate 18% Example 6Desmophen 2000 42.5% Multranol 9144 31.7% Desmodur W 19.8% DBTDL 0.02%Zinc Borate 6%

[0082] It is well known to those versed in the techniques ofsuccessfully producing polyurethane films and other products (with theexception of foams) that it is essential to exclude water from theingredients or the end result will be that soft and weak materials whichare full of bubbles. The reason for this is that isocyanates react withwater to produce carbon dioxide which ends up as bubbles in the finalproduct. For this reason a variety of techniques have been developed tokeep water out of the reactants.

[0083] The most common way for water to contaminate the reactants is forit to be absorbed by them from the surrounding air. Unless the air isdried to an extremely low moisture content (for example a relativehumidity of 10% or lower) the polyols used for the preparation of thepolyurethanes will absorb sufficient water vapor from the moist air toproduce an inferior product. With regard to the isocyanate component ofthe formulations the reaction with water referred to above not onlyproduces the deleterious bubbles, but it also “weakens” the isocyanateso that the stoichiometry is thrown off balance, and a soft weak productresults. It is well known that in order to produce strong high molecularweight polyurethane polymers, equimolar amounts of hydroxyl and urethanegroups must be present in the reaction mixture.

[0084] Since in the presence of a polymerization catalyst (metal salts,amines, etc.) the hydroxyl and isocyanate materials will react to form apolyurethane, it is essential that these two components be keptseparately until the time that they are purposely allowed to react toform the final desired product.

[0085] As was stated above a variety of techniques have been developedto keep water from the reactants. If the polyols and isocyanates arepurchased commercially, they are shipped in sealed containers and areblanketed by an atmosphere of dry nitrogen. When these containers areopened for use the must be opened in a dry atmosphere, or the localsurrounding environment must be opened in a dry nitrogen (or air) duringthe pouring or transfer operation. The materials must be transferredinto containers which have been dried, and which have a dry atmosphere.After the reactants have been transferred to these containers, a dryatmosphere must be maintained by flooding with dry gas before closingand sealing. The opened containers from which the reactants have beenpoured must also be flooded with dry gas before resealing. One techniquewhich has often been used successfully is to transfer the materials fromone container to another by the use of vacuum. A slight vacuum is drawnin the receiving container, and the material is transferred from thestorage container to the receiver by “blowing” it from the one vessel tothe other by taking advantage of the pressure differential.

[0086] By the use of the various techniques just described, the productsgiven in the above examples are prepared as follows: The polyolcomponent (A component) is prepared by mixing together the polyols givenin each example along with the polymerization catalyst (DBTDL in theexamples given) and the chromate corrosion inhibitors. If the materialis to be sprayed to form a thin film, the chromates must be milled intoa polyol component using a suitable mill such as a ball mill, sand millor three roll mill to a paste in which the pigment is finely dispersed.Generally milling the paste to a Hegmen gauge reading of 6 or higherresults in a satisfactory dispersion. Care must be taken during themilling process to ensure that no water is allowed to contaminate thepaste.

[0087] The isocyanate (B component) is weighed in the proper amount, andthe A and B components are mixed together just prior to application. Themixed product is allowed to polymerize to cure to the final product. Inthe examples given an overnight cure will result in a satisfactorymaterial. The cure can be slowed up or hastened by adjusting the amountor type of catalyst. For example the cure can be slowed down bydecreasing the amount of tin catalyst used, or by, substituting an aminecatalyst for the tin catalyst. The cure can be accelerated by increasingthe amounts of tin catalyst, using a different organo tin salt (forexample dibutyl tin dichloride) or by using an amine catalyst along withthe DBTDL.

[0088] The final mixed product is applied to the substrate by a suitabletechnique such as brushing, spraying, drawing down a film, trowellingetc. As stated above spraying is usually the preferred method ofapplication.

[0089] Although the preferred polymers to use for this invention arepolyrethanes, any polymer which can be formulated to give a softflexible material with the correct physical properties to show adequatesolvent resistance, temperature resistance, etc., as described above canbe used. Satisfactory useful polymer types in addition to thepolyurethanes include polyesters, epoxies, acrylics, silicones, naturaland synthetic rubbers, polybutadienes and certain vinyl materials.

[0090] This invention is not to be limited by the embodiments shown inthe drawings and described in the description, which are given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims. CUSTOMER NUMBER 24201 RUN DATE MAR. 28, 2001CUSTOMER NUMBER ASSIGNMENTS TO PATENTS AND PENDING APPLICATIONS FULWIDERPATTON LEE & UTECHT, LLP HOWARD HUGHES CENTER 6060 CENTER DRIVE TENTHFLOOR LOS ANGELES CA 90045 PRAC- APPLICATION FILING PATENTCORRESPONDENCE TITIONER FEE NUMBER PATENT NUMBER DATE DATE ADDRESS OFRECORD ADDRESS 00000000 4408936 Oct. 11, 1983 YES YES YES ACCEPTEDDONALD D. MON PRIOR CORRESPONDENCE ADDRESS NO PRIOR FEE ADDRESS 08840099JEFFREY F. CRAFT Apr. 11, 1997 YES YES YES ACCEPTED PRIOR CORRESPONDENCEADDRESS NO PRIOR FEE ADDRESS 00000000 4815907 Mar. 28, 1989 YES YES YESACCEPTED LAURENCE H. PRETTY PRIOR CORRESPONDENCE ESQ. PRETTY, ADDRESSPRIOR FEE SCHROEDER & POPLAWSKI ADDRESS 00000000 5485762 Jan. 23, 1996YES YES YES ACCEPTED DONALD MON PRIOR CORRESPONDENCE PRETTY, SCHROEDERADDRESS PRIOR FEE & POPLAWSKI ADDRESS 09198741 JEFFREY F. CRAFT Nov. 24,1998 YES YES YES ACCEPTED PRIOR CORRESPONDENCE ADDRESS NO PRIOR FEEADDRESS 00000000 6133371 Oct. 17, 2000 YES YES YES ACCEPTED JEFFREY F.CRAFT PRIOR CORRESPONDENCE ADDRESS NO PRIOR FEE ADDRESS 00000000 6085940Jul. 11, 2000 YES YES YES ACCEPTED PETER A BORSARI PRIOR CORRESPONDENCEADDRESS NO PRIOR FEE ADDRESS 09236576 PETER A BORSARI Jan. 26, 1999 YESYES YES ACCEPTED PRIOR CORRESPONDENCE ADDRESS NO PRIOR FEE ADDRESS09613587 Peter A Borsari Jul. 10, 2000 YES YES YES ACCEPTED PRIORCORRESPONDENCE ADDRESS NO PRIOR FEE ADDRESS

I claim:
 1. A sealant for forming a fluid seal between the fayingsurface of two metallic substrates that are compressively andmechanically joined together, said sealant being liquid in itspure-cured condition and a dry solid in its cured condition, thetransition from pre-cured to cured condition occurring at roomtemperature with a cure time not less than about 10 minutes and not morethan about 16 hours, said sealant when cured having a Shore A hardnessbetween about 30 and about 70, being impermeable and resistant tochemical attack by air, water, common solvents and petroleum fuels,resistant to compressive cold flow but deformable to conform to anabutting one of said surfaces or with a layer of similar sealant on theother of said surfaces, and flexible and resistant to a temperaturerange between about −65 degrees F. and about 250 degrees F., saidsealant comprising a cured copolymer of an isocyanate and a polyol, saidsealant including a metal borate as a corrosion resistance component. 2.A sealant according to claim 1 in which both the isocyanate and polyolhave at least two functionalities.
 3. A sealant according to claim 2 inwhich the isocyanate and polyol when mixed before curing contain acatalyst in an amount respective to an intended cure rate.
 4. A sealantaccording to claim 3 in which the catalyst is an organic salt of tin ormercury.
 5. A sealant according to claim 4 in which the catalyst isDBTDL.
 6. A sealant according to claim 2 in which the isocyanate andpolyol when mixed before curing contains said metal borate.
 7. A sealantaccording to claim 6 in which the metal borate is zinc borate in amountsby weight between about 3% and about 30% of the sealant.
 8. A sealantaccording to claim 7 in which the zinc borate is in amounts betweenabout 6% and about 12% by weight of sealant.
 9. A structural assemblycomprising a pair of substrates each having a faying surface, saidfaying surfaces facing toward one another; a layer of sealant betweensaid faying surfaces and adherent to at least one of them; and fastenerscompressively holding said surfaces against said sealant layer to form aseal between said surfaces; said sealant comprising a sealant forforming said seal between said faying surfaces said sealant being liquidin its pre-cured condition and a dry solid in its cured condition, thetransition from pre-cured to cured condition occurring at roomtemperature with a cure time not less than about 10 minutes and not morethan about 16 hours, said sealant when cured having a Shore A hardnessbetween about 30 and about 70, being impermeable and resistant tochemical attack by air, water, common solvents and petroleum fuels,resistant to compressive cold flow but deformable to conform to anabutting one of said surfaces or with a layer of similar sealant on theother of said surfaces, flexible and resistant to a temperature rangebetween about −65 degrees F. and about 250 degrees F., said sealantcomprising a cured copolymer of an isocyanate and a polyol, said sealantincluding a metal borate as a corrosion resistance component.
 10. Asealant according to claim 9 in which both the isocyanate and polyolhave at least two functionalities.
 11. A sealant according to claim 10in which the isocyanate and polyol when mixed before curing contain acatalyst in an amount respective to an intended cure rate.
 12. A sealantaccording to claim 11 in which the catalyst is an organic salt of tin ormercury.
 13. A sealant according to claim 12 in which the catalyst isDBTDL.
 14. A sealant according to claim 10 in which the isocyanate andpolyol when mixed before curing contains said metal borate.
 15. Asealant according to claim 14 in which the metal is zinc borate inamounts by weight between about 3% and about 30% of the sealant.
 16. Asealant according to claim 14 in which the zinc borate is in amountsbetween about 6% and about 12% of the sealant.